The present invention relates to an exhaust gas recirculation system (EGR system) for internal combustion engines. More specifically, the present invention is directed to EGR systems of the type which recirculate at least a portion of the engine exhaust gases into the engine air intake system for the purpose of reducing NOx emissions.
With continued tightening of governmental regulations on vehicle exhaust emissions, particularly NOx emissions, not only has the need to recirculate exhaust gases back into the engine intake become apparent, but so has the need to improve upon existing exhaust gas recirculation technology.
U.S. Pat. No. 4,217,869 issued to Masaki discloses an exhaust gas recirculation system in which combustion gases are forced from a reaction chamber through an outlet port into an intake passageway by either an ejector effect or suction produced by the engine exhaust gases drawn from an outlet portion of an EGR passageway. However, such a system when used on engines having efficient turbo machinery and/or an EGR cooler, especially on medium to heavy-duty diesel engines, face the problem that an exhaust-to-intake pressure differential can occur that is either too low or too high thus resulting in an inefficiency in the exhaust gas recirculation. This is particularly the case at rated speed and high loads where the EGR rates near 20% may be required, necessitating EGR flow rates beyond that which simple venturi or ejector aided induction systems can supply.
Future automotive medium-duty and heavy-duty engines will require cooled, recirculated exhaust gases in order to comply with the NOx emissions standards which at the present time are projected at 2 grams per BHP-/hr. Present engines are generally not capable of exhaust gas recirculation delivery because of their highly efficient turbochargers which develop a positive pressure difference. That is, the intake pressure exceeds exhaust pressure over much of the engine operating range. Various measures to provide exhaust gas recirculation have been investigated including compounding, waste gates, as well as variable geometry turbochargers. However, significant shortcomings are associated with each of these alternatives individually.
While compounding is an effective means of developing a negative pressure differential with which to drive the exhaust gas recirculation system, it has significant drawbacks. Most notably are the complexity of the system which leads to difficulties in packaging the system and consequently results in a significant increase in the overall manufacture and cost of the engine.
The deficiencies of pressure differential type EGR induction systems have been recognized for some time. In U.S. Pat. No. 4,196,706 issued to Kohama et al., control valves are used to regulate the quantity of exhaust gas that is recirculated, and in recognition of the fact that insufficient EGR pressure may exist under certain operating conditions, U.S. Pat. No. 4,276,865 issued Hamai teaches the use of an engine driven pump upstream of the EGR control valve for ensuring that sufficient pressure exists to introduce the EGR gases into the engine intake passageway. However, the use of an engine driven pump adds to the cost and weight of the EGR system, and is thus a source of parasitic losses.
In an effort to overcome the aforementioned shortcomings, a system developed by the assignee of the subject application, Cummins Engine Company, Inc., set forth in U.S. Pat. No. 5,611,203 issued to Henderson et al. discloses an ejector pump enhanced high pressure EGR system. This system utilizes an ejector which is provided with mixer lobes and a defuser which enhances the momentum transfer from the intake flow to the exhaust flow to introduce the EGR exhaust gas flow into the intake passageway. In this manner, the static pressure of the exhaust flow at the entrance of the mixing region is decreased, thereby increasing the differential pressure across the EGR tube and increasing the exhaust flow. As an alternative, the ejector in the EGR tube may be connected to the vehicle air system compressor or turbo compressor which serves to pump the exhaust gases to the engine intake passageway. In either case, such a system requires the use of high pressure air in order to efficiently operate the exhaust gas recirculation system.
Accordingly, there is a need for an exhaust gas recirculation system which provides high engine efficiency when used in connection with non-compound turbo charged engine systems such as non-compound medium-duty and heavy-duty engines which accumulate high annual mileage.
A primary object of the present invention is to overcome the aforementioned shortcomings associated with prior exhaust gas recirculation systems.
A further object of the present invention is to provide an exhaust gas recirculation system utilizing in combination a variable geometry turbine and wastegate to allow the scheduling of both air flow and exhaust gas recirculation flow rates at any speed and load.
The system set forth in accordance with the present invention allows minimum air flow and accurate control of air flow and exhaust gas recirculation flow.
A further object of the present invention is to provide a system which minimizes the air flow which equates to a minimization of exhaust gas recirculation flow.
A still further object of the present invention is to minimize the exhaust gas recirculation flow so as to minimize the exhaust gas recirculation cooler size thus reducing cost and heat rejection.
A still further object of the present invention is to provide an exhaust gas recirculation-air handling system which minimizes the negative pressure differential across the engine necessary to drive the exhaust gas recirculation system thus resulting in better fuel economy.
A still further object of the present invention is to provide an exhaust gas recirculation system wherein fuel economy is increased by eliminating the need to throttle the engine during operation.
Yet another object of the present invention is to provide an exhaust gas recirculation system wherein the air flow rate and exhaust gas recirculation rate are controlled independently of one another to ensure maximum efficiency at various engine operating conditions and environmental conditions.
These, as well as additional objects of the present invention are achieved by providing in combination a variable geometry turbine and active wastegate while further providing independent control of the variable geometry turbine and wastegate valve so as to independently control exhaust gas recirculation flow and air flow in an internal combustion engine. Such independent control of exhaust gas recirculation flow and air flow in an internal combustion engine is achieved by providing a system for exhaust gas recirculation including an intake air manifold, an exhaust manifold, an exhaust gas recirculation passage providing fluid communication between the exhaust manifold and the intake air manifold, a variable geometry turbine positioned downstream of the exhaust manifold, and a waste gate passage providing fluid communication between the exhaust gas manifold and a point downstream of the variable geometry turbine. A control system is provided for controlling exhaust gas recirculation through the exhaust gas recirculation passage by independently controlling a geometry of the variable geometry turbine and thus the casing size of the turbine to control air flow and passage of exhaust gas through a waste gate passage in order to control compressor power which delivers the pressure ratio to the compressor and intake manifold. In doing so, both flow and power are independently controlled in order to maximize efficiency of the system.
These, as well as additional objects of the present invention, will become apparent from the following detailed description when read in light of the several figures.